1. Field of the Invention
The present invention relates to an ink set and a method of recording a color image on a recording material by using a combination of a plurality of inks for primary colors. In particular, the invention relates to a set of inks and an ink jet recording method which permit the formation of a sharp and high-quality image having a sufficient density.
2. Description of the Related Art
An ink jet recording method employs one of various ink discharging means for generating ink droplets including electrostatic suction means for applying a high voltage to ink, means for applying mechanical vibrations or displacement to coloring ink by using a piezoelectric element, means for heating ink to form bubbles by applying pressure thereto, and so forth. Upon recording, the ink droplets generated are ejected to adhere to a recording material such as recording paper or the like and form ink dots. This recording method causes less noise and enables high-speed printing and multicolor printing.
The printed images recorded by this ink jet recording method are used for various applications, such as observation of a surface image recorded on a recording material such as paper, ink jet recording paper or the like, or observation of an image recorded on a light-transmitting recording material by projecting the image on a screen or the like using an optical apparatus such as a slide, OHP (Over Head Projector) or the like. The recorded images are also used for a color separation plate for forming a color printing positive plate; CMF (Color Mosaic Filter) used in a liquid crystal color display and the like.
On the other hand, a typical example of recording processes used for forming color images by such an ink jet recording method is a subtractive color mixture process.
When a color image is formed by the subtractive color mixture process, the three primary colors of cyan (C), magenta (M) and yellow (Y) are generally used.
Colors which can be produced by mixing the primary colors in the same pixel, i.e., superposing ink droplets having different colors in the same pixel, include the following four colors:
(1) C+M=B (blue) PA0 (2) C+Y=G (green) PA0 (3) M+Y=R (red) PA0 (4) C+M+Y=Bk (black)
The colors which can be produced by the primary colors C, M and Y are basically seven colors including C, M, Y, R, G, B and Bk. A multicolor image having two or more of the seven colors is referred to as a "false color image".
Thus, when an image is recorded by this recording method, inks for the primary colors C, M and Y may be prepared.
However, when the color Bk is produced by the primary colors of C, M and Y, the color Bk is printed by superposing ink droplets of the primary colors C, M and Y. In this case, since the amounts of the ink droplets of the respective primary colors are substantially the same, the ink droplets of the three colors in a Bk portion are superposed and adhere to a recording material, and the dot formed is excessively enlarged, as compared with a dot formed by each of the colors other than Bk, which comprises only one or two of the three primary colors. This causes the problem of line thickening and thus forming an unnatural image, or the problem that the ink is not sufficiently absorbed by the recording material because large amounts of the inks are used. Since an image is frequently recorded with the color Bk, these problems are very important.
In order to overcome the problems, inks of the three primary colors C, M and Y are generally used, and a fourth ink of the color Bk is used for producing the color Bk.
When inks of the four colors C, M, Y and Bk are used for recording, however, since ink droplets of two different colors are superposed in a printed portion of each of the colors B, G and R, as described above, if a recording material is composed of wood free paper containing a sizing agent, a large duration of time is required for absorbing large amounts of ink droplets. Consequently, the ink droplets enter gaps between the fibers of the paper not only in the sectional direction of the recording material but also in all other directions thereof during the absorption over the large time duration. This causes the formation of nonuniform dots and line thickening, i.e., so-called feathering. There is thus the problem of recording an unclear image without edge sharpness.
In addition, even when ink droplets having two different colors are placed adjacent to each other, the unfixed portions of the ink droplets are mixed at the boundaries therebetween, thereby causing bleeding at the boundaries between different colors and thus resulting in an unclear image.
A possible method of solving the above problems is to increase the speed of penetration of ink droplets into the recording material. This method may decrease the surface tension of each color ink and thus realize an improvement in the wettability of the recording material by the ink.
However, when the surface tension of each ink is decreased, the dots formed on the recording material are excessively enlarged, thereby causing line thickening. Alternatively, an increase in the penetration causes the coloring materials to deeply penetrate into the recording material, and thus decreases the color density.
As described above, the color Bk is frequently used for characters. If a character of the color Bk could have a higher color density than that of a character of each of the other colors, and could further have a sharp edge without unnatural thickening, the character so formed would have a preferred appearance.